The present invention relates to a process for manufacturing needle coke from substantially ashless liquefied coal solutions.
Coke, as the material for electrodes, is presently manufactured from mainly petroleum heavy oil or coal tar pitch. The properties required of such cokes include, among others, suitable strength and specific gravity, an acceptable level of impurities and the proper crystalline structure. With regard to the crystalline structure, some cokes are rich in amorphous substances, while others are rich in needle-like crystals, the former being suitable for producing electrodes for use in aluminum refining, and the latter being more suitable for producing large-sized artificial graphite electrodes.
A large-sized artificial graphite electrode requires low electrical resistance, a small coefficient of thermal expansion, high density and a high level of physical strength, in, order to cope with the operation of a steel-making electric furnace. The material therefor, consequently, should contain a large quantity of the so-called needle coke having a needle-like crystalline structure of easily and readily graphitizable nature.
Hitherto needle coke was manufactured from coal tar pitch, as the specifically suited material; however, there is an insufficiently limited supply of coal tar pitch for the high demand of modern industry. Technological efforts for manufacturing needle coke as well as amorphous coke from heavy fractions of petroleum are presently being conducted, but the presence of abnormally large quantities of impurities, including 300-500ppm of vanadium, 100-500ppm of nickel, and approximately 1000ppm of Fe+Si, in the material still remains a mostly unresolved problem in using heavy petroleum fractions as the source materials.
Ashless coke, on the other hand, manufactured from substantially ashless liquefied coal, that is, manufactured by the processes of solvent extraction of coal or coal hydrogenation has been reported to be exclusively of the amorphous type. More specifically, around 1940, coal was subjected to extraction for refining by the use of fractions of coal tar, and substantially ashless liquefied coal was produced, such ashless coke being found suitable as material for carbon electrodes for electrolysis of aluminum. This was however amorphous coke having properties suitable only for making electrodes to be employed for electrolysis of aluminum.